Many enclosures that contain sensitive instrumentation must maintain very clean environments in order for the equipment to operate properly. Examples include enclosures with optical surfaces or electronic connections that are sensitive to particles and gaseous contaminants which can interfere with mechanical, optical, or electrical operation. Other examples include data recording devices such as computer hard disk drives that are sensitive to particles, organic vapors, and corrosive vapors. Still others include enclosures for processing, transporting or storing thin films and semiconductor wafers. Also included are electronic control boxes such as those used in automobiles and industrial applications that can be sensitive to particles, moisture buildup, and corrosion as well as contamination from fluids and vapors. Contamination in such enclosures originates from both inside and outside the enclosures. For example, in computer hard drives, damage may result from external contaminates as well as from particles and outgassing generated from internal sources. The terms “hard drives” or “hard disk drives” or “disk drives” or “drives” will be used herein for convenience and are understood to include any of the enclosures mentioned above.
To address contamination problems, internal particulate filters, or recirculation filters, are installed in disk drives. These filters may incorporate filter media, such as expanded PTFE membrane laminated to backing material such as a polyester nonwoven, or “pillow-shaped” filters containing electret (i.e., electrostatic) filter media or triboelectret media. Electret and triboelectret media are collectively described herein as “electret media”. They may be pressure fit into slots or “C”-shaped channels and placed into the active air stream such as near the rotating disks in a computer hard disk drive or in front of a fan in electronic control cabinets, etc. These filters may have cover layers to contain fibers, increase stiffness, and generally improve handling or usability of the filter. Alternatively, the recirculation filter media can be framed in a plastic frame.
Recirculation filters for computer hard disk drives may also consist of a layer of electret media with one or more layers of scrim on either side of the electret layer. The outer scrim layer or layers are used to contain the fibers of the electret layer as well as add stiffness for ease of handling, weldability and the like.
Filter performance has been known to be a function of filter material weight. Higher weight per square meter materials have both a higher efficiency and a higher pressure drop. Electret filter layers are often specified by two parameters: the weight per unit area of electret fibers needled into a scrim, and the weight of the scrim. A typical scrim weight is 15 grams per square meter, but others are available. Common electret media weights may be from about 70 grams per square meter to about 90 grams per square meter, although other material weights are available. Other electret layers may be scrimless electret layers or entangled electret fibers.
One theory used to predict filter performance is Quality Factor. Quality Factor is described in Air Filtration by R. C. Brown, Paragon Press, 1993. Quality Factor (Qf) is defined as:Qf=−In(penetration)/pressure drop
Penetration is defined as the ratio of particles passing through the media to the total number of challenge particles. The inventors have discovered that while penetration and pressure drop are important to filter performance, filter thickness is also unexpectedly important.
Accordingly, the present invention provides an improved electret recirculation filter that can better filter the air of particles to better prevent particle problems inside the drive and increase drive reliability.